KCl Background Research Articles

The influence of water acidity and alkalinity on the adhesion properties of bitumen-aggregate interface

To examine the influence of water acidity and alkalinity on bitumen-aggregate interfacial adhesion, we measured interaction force between aggregate and bitumen using atomic force microscopy with a 1 mmol/L KCl background solution at pH levels ranging from 2 to 10. Experimental materials included four types of aggregates (basalt, granite, limestone A, and limestone B) and two types of bitumen (Zhenhai 90# matrix and Styrene-Butadiene-Styrene modified). Results showed adhesion rankings: granite < basalt < limestone B < limestone A. Long-range forces increased with higher pH, in line with Derjaguin-Landau-Verwey-Overbeek theory. Acidic conditions reduced repulsive forces between bitumen and aggregates while increasing adhesive forces. Compared with the Zhenhai 90# matrix bitumen, Styrene-Butadiene-Styrene modified bitumen demonstrated superior adhesion. Zeta potential values for bitumen and aggregate consistently decreased with rising pH, reflecting the aggregate-bitumen interaction force, suggesting electrostatic forces' dominance. Boiling water method was used to assess bitumen spalling on different aggregates under varied acid-base conditions, finding a significant correlation between spalling rate and microscopic interaction force. This study offers theoretical support for minimizing water damage in bitumen pavements by shedding light on the adhesion process of bitumen and aggregates in wet settings.

The Cumulative Amount of Exuded Citrate Controls Its Efficiency to Mobilize Soil Phosphorus

Root exudation of citrate is discussed as mechanism to mobilize P from the soils' solid phase. Microbial processes can mitigate the mobilization efficiency of citrate. Due to higher microbial activity in topsoils compared to subsoils, we hypothesized a lower mobilization efficiency of exuded citrate in topsoils than in the subsoils. As a model system we used microdialysis (MD) probes and we followed diffusive fluxes of citrate from the perfusate into the soil and of phosphate from the soil into the dialysate in three soil horizons (Oa, Ah, Bw) of a Fagus sylvatica L. stand Cambisol. Three different MD perfusates with a KCl background concentration have been used: control, 1, and 3 mmol L−1 citric acid. Fluxes have been measured after 24, 48, and 144 h. The high-citrate perfusate increased the cumulative 144 h P-influx by a factor of 8, 13, and 113 in the Oa, Ah, and Bw horizon, respectively. With the high-citrate treatment, P mobilization efficiency decreased over time, whereas for the low citrate, P mobilization efficiency had a maximum at day 2. Minimum P mobilization efficiency of citrate was 1:25,000 mol phosphate per mol citrate in the Oa horizon between days 2 and 6, and maximum was 1:286 in the Bw-horizon during day 2. An increasing citrate efflux over time indicated an increasing sink term for citrate in the soil due to microbial decay or immobilization processes. Cumulative phosphate influx could be fitted to cumulative citrate efflux and soil horizon in a logarithmic model explaining 87% of the variability. For the first time, we could follow the localized P-uptake with citrate exudation over several days. Cumulative citrate efflux as the main control of P-mobilization has been barely discussed yet, however, it could explain some gaps in the role of carboxylates in the rhizosphere. Batch experiments are not capable to elucidate microscale dynamic competition for phosphate and carboxylates. MD is a promising tool for spatially explicit investigation of phosphate–citrate exchange, since such detailed insights in are not possible with batch experiments. In combination with the analysis of microbial properties, this technique has a huge potential to identify mobilization processes in soils as induced by citrate.

Fluorous-Phase Ion-Selective pH Electrodes: Electrode Body and Ionophore Optimization for Measurements in the Physiological pH Range.

Because of their low polarity and polarizability, fluorous sensing membranes are both hydrophobic and lipophobic and exhibit very high ion selectivities. Here, we report on a new fluorous-membrane ion-selective electrode (ISE) with a wide sensing range centered around physiologically relevant pH values. The fluorophilic tris[perfluoro(octyl)butyl]amine (N[(CH2)4Rf8]3) was synthesized and tested as a new H+ ionophore using a redesigned electrode body that provides excellent mechanical sealing and much improved measurement reliability. In a challenging 1 M KCl background, these fluorous-phase ISEs exhibit a sensing range from pH 2.2 to 11.2, which is one of the widest working ranges reported to date for ionophore-based H+ ISEs. High selectivities against common interfering ions such as K+, Na+, and Ca2+ were determined (selectivity coefficients: logKH, Kpot = – 11.6; logKH, Napot = – 12.4; logKH, Capot < – 10.2). The use of the N[(CH2)4Rf8]3 ionophore with its −(CH2)4– spacers separating the amino group from the strongly electron-withdrawing perfluorooctyl groups improved the potentiometric selectivity as compared to the less basic tris[perfluoro(octyl)propyl]amine ionophore. The use of N[(CH2)4Rf8]3 also made the ISE less prone to counter anion failure (i.e., Donnan failure) at low pH than the use of tris[perfluoro(octyl)pentyl]amine with its longer −(CH2)5– spacers, which more effectively shield the amino center from the perfluorooctyl groups. In addition, we exposed both conventional plasticized PVC-phase pH ISEs and fluorous-phase pH ISEs to 10% serum for 5 days. Results show that the PVC-phase ISEs lost selectivity while their fluorous-phase counterparts did not.

Dissolution and Mobilization of Uranium in a Reduced Sediment by Natural Humic Substances under Anaerobic Conditions

Biological reduction and precipitation of uranium (U) has been proposed as a remedial option for immobilizing uranium at contaminated sites, but the long-term stability and mobility of uranium remain a concern because the uranium is neither removed nor destroyed. In this study, the dissolution and mobilization of reduced and oxidized forms of uranium [U(IV) and U(VI)] by natural humic substances were investigated in batch and column-flow systems using a bioreduced sediment containing both U(IV) and U(VI). The addition of humic substances significantly increased the dissolution of U(IV) under anaerobic conditions. Humic acid (HA) was found to be more effective than fulvic acid (FA) in dissolving U(IV) in 1 mM KCl or KHCO3 background solution. However, more U(VI) was dissolved in 1 mM KHCO3 than in 1 mM KCl background electrolyte. HA also was found to be more effective than FA in mobilizing uranium under reducing and column-flow conditions, although the cumulative amount of eluted U(VI) and U(IV) was relatively low (<60 microg) after leaching with approximately 97 pore volumes of the humic solution in 1 mM KHCO3. These observations suggestthat natural humic substances could potentially influence the long-term stability of bioreduced U(IV) even under strongly reducing environments.

Acidity constants in different media (I=0 and I=0.1 M KCl) from the uncertainty perspective

Procedures for estimating the measurement uncertainty for the acidity constant Ka (or the pKa value) in different media (I=0 and I=0.1 mol L(-1) KCl), as determined by potentiometric titration, are presented. The uncertainty budgets (the relative contributions of the different input quantities to the uncertainty in the result) of the pKa (I=0) and pKa (I=0.1 mol L(-1) KCl) values are compared. Unlike the values themselves, the uncertainties and uncertainty budgets of the values are comparable. The uncertainty estimation procedures are based on mathematical models of pKa measurement and involve the identification and quantification of individual uncertainty sources according to the ISO GUM approach. The mathematical model involves 52 and 48 input parameters for pKa (I=0) and pKa (I=0.1 mol L(-1) KCl), respectively. The relative importance of each source of uncertainty is discussed. In both cases, the main contributors to the uncertainty budget are the uncertainty components due to the hydrogen ion concentration/activity measurement, which provide 63.7% (for pKa (I=0)) and 89.3% (for pKa (I=0.1 mol L(-1) KCl)) of the uncertainty. The remaining uncertainty contributions arise mostly from the limited purity of the acid. From this work, it is clear that the uncertainties of the pKa (I=0.1 mol L(-1) KCl) values tend to be lower than those of the pKa (I=0) values. The main reasons for this are that: (1) the uncertainty due to the residual liquid junction potential is nominally absent in the case of pKa (I=0.1 mol L(-1) KCl) due to the similarly high concentrations of background electrolyte in the calibration solutions and measured solution; (2) the electrode system is more stable in solutions containing the 0.1 mol L(-1) KCl background electrolyte and so the readings obtained in these solutions are more stable.

Application of a New pH-Sensitive Electrode as a Detector in Flow Injection Potentiometry

A new pH sensitive detector for flow-through potentiometry was developed on the basis of a graphite/quinhydrone composite electrode. The detector was constructed of two polymethylmethacrylate plates between which a 0.1 mm thick sensitive layer is situated. After drilling a hole through the plates, a ring of the sensitive layer is exposed to the solution stream. A conventional calomel reference electrode was placed downstream following the flow-through sensor. The response behavior in different electrolyte solutions was investigated. The detector shows a Nernstian behavior for injections of hydrochloric acid into a KCl background solution as well as for the steady state response in concentration step experiments using hydrochloric acid and buffer solutions. The response time is sufficiently short for FIA applications (T95 between 3 and 5.4 s). The influences of flow rate and injection volume on the detector signal are discussed.

Kinetics and Isotherms of Cationic Polyelectrolyte Adsorption on Quartz

The kinetics of the formation of quartz surface charge in the solutions of a cationic polyelectrolyte, poly(styrene-co-dimethyl aminopropylmaleimide) with the molecular mass M = 20 000 is studied in the concentration range from 10–5 to 0.5 g/l in 10–4 M KCl background solution at pH 6.5. Quartz capillaries with the radius from 5 to 10 μm and molecularly smooth surface are used as model systems. Characteristic times of the formation of the surface charge at equilibrium with the solution are calculated from the data on the kinetics of adsorption; these times are equal to 40–50 min for the region of electrostatic adsorption (before the surface charge reversal) and 20–25 min, for the region of hydrophobic adsorption upon the formation of the second adlayer. Based on the steady values of the surface charge, the isotherms and potentials of adsorption of cationic polyelectrolyte are calculated. Electrostatic adsorption isotherm is described by the Langmuir equation with the energy of molecular adsorption of 25.4kT. It is shown that, at polymer concentration above 10–2 g/l, the conformation of adsorbed molecules ceases to be planar. However, even in this case, we succeed in calculating the surface charge using the Helmholtz and Gouy equations and applying the pressure drops at the capillary ends higher than 10 atm, when under the loading of increasing shear stress in the surface layer the conformation of adsorbed molecules approaches the planar shape. Based on the two-layer model of the formation of surface charge developed earlier, it is shown that the energy of hydrophobic adsorption is smaller than that of electrostatic adsorption and is equal to 17.7kT. Possible physical mechanisms of electrostatic and hydrophobic adsorption of cationic polyelectrolyte molecules on quartz are discussed.

Kinetics of electron transfer between Fe(CN) 63−/4− and poly(3,4-ethylenedioxythiophene) studied by electrochemical impedance spectroscopy

The electron transfer between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and the Fe(CN) 6 3−/4− redox couple in aqueous solution was investigated by electrochemical impedance spectroscopy (EIS). PEDOT was electrochemically deposited on platinum from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M poly(sodium 4-styrenesulfonate) (NaPSS) as supporting electrolyte. Pt/PEDOT(PSS) electrodes with polymer films of different thickness were investigated at different concentrations of the redox couple in 0.1 M KCl background electrolyte solution. Impedance spectra were obtained at the dc-potential corresponding to the formal redox potential of Fe(CN) 6 3−/4− ( E°′≈220 mV) where the polymer is in the oxidized and electrically conducting state. The EIS data for the electrodes were fitted to an equivalent electrical circuit. The standard rate constant ( k o) for electron-transfer between Pt/PEDOT(PSS) and Fe(CN) 6 3−/4− was determined by calculations based on the Butler–Volmer equation. The diffusion coefficient ( D) of the redox couple was also calculated from the EIS data.

Electrosurface Properties of Hydr(oxides) and Oxide Nanostructures in 1 : 1 Electrolyte Solutions: 2. Electrokinetic Characteristics of Boehmite, Goethite, and Silicon Dioxide

The electrophoretic mobility of particles of silicon dioxide, aluminum hydroxide, and iron hydroxide was measured as a function of pH in NaCl and KCl background solutions. Isoelectric points for the investigated objects were determined. Electrokinetic potentials were calculated with allowances for the particle shape and the polarization of the electrical double layer within the framework of the Overbeek–Boes–Wiersema model.

Surface catalytic hydrogen evolution in polarography of nonbuffer pyridine solutions

1. The current strength vs. time curve during growth of a single drop, taken on the first drop of a mercury drop electrode in nonbuffer pyridine solutions on a KCl background has been used to find the surface component of the limiting catalytic current, the values of which have been used to calculate the rate constants for protonation of pyridine by water molecules when the pyridine is in the adsorbed state. 2. The rate constant for protonation of adsorbed pyridine by water is more than an order of magnitude greater than the volume pyridine protonation constant. The value of the surface constant decreases with increase in ionic strength of the solution.